Carbon Nanotube-Graphene Hybrid Transparent Conductor and Field Effect Transistor

ABSTRACT

A nanotube-graphene hybrid film and method for forming a cleaned nanotube-graphene hybrid film. The nanotube-graphene hybrid film includes a substrate; nanotube film deposited over the substrate to produce a layer of nanotube film; and graphene deposited over the layer of nanotube film to produce a nanotube-graphene hybrid film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/301,943, filed Nov. 22, 2011, which is expressly incorporated byreference herein.

FIELD OF THE INVENTION

Embodiments of the invention generally relate to electronic devices and,more particularly, to transparent conductors.

BACKGROUND OF THE INVENTION

The ever-increasing cost of indium, a rare earth element used incoatings for transparent conductors (TCs), has given an impetus forresearch into several alternatives for use as a TC. Indium tin oxide(ITO) and other amorphous TCs (fluorine doped zinc oxide, etc.) alsohave problems with use in the flexible electronic industry.

In recent years, carbon nanotube films and graphene have emerged aspotential replacements for ITO as a TC in various technologicalapplications such as photovoltaic devices, electronic displays, etc.However, these alternatives still fall short of the required performancefor TC coatings in terms of transparency and sheet resistance.Accordingly, a need exists for effective alternatives for TC coating.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method for forming a cleanednanotube-graphene hybrid film is provided. The method includes the stepsof depositing nanotube film over a substrate to produce a layer ofnanotube film, removing impurities from a surface of the layer ofnanotube film not contacting the substrate to produce a cleaned layer ofnanotube film, depositing a layer of graphene over the cleaned layer ofnanotube film to produce a nanotube-graphene hybrid film, and removingimpurities from a surface of the nanotube-graphene hybrid film toproduce a cleaned nanotube-graphene hybrid film, wherein the hybrid filmhas improved electrical performance via decreasing nanotube resistanceby increasing contact area through use of graphene as a bridge.

Another aspect of the invention includes a nanotube-graphene hybridfilm. The hybrid film includes a target substrate, a layer of nanotubefilm deposited over the target substrate, and a layer of graphenedeposited over the layer of nanotube film.

Yet another aspect of the invention includes a method for forming ananotube-graphene hybrid film on a substrate. The method includes thesteps of depositing nanotube film over a metal foil to produce a layerof nanotube film, placing the metal foil with as-deposited nanotube filmin a chemical vapor deposition furnace to grow graphene on the nanotubefilm to form a nanotube-graphene hybrid film, and transferring thenanotube-graphene hybrid film over a desired substrate.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example schematic of the carbonnanotube-graphene composite, according to an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating two-dimensional (2D) resistor latticesas a model system for a CNT-graphene hybrid, according to an embodimentof the present invention;

FIG. 3 is a diagram illustrating a transmittance and sheet resistancedata comparison, according to an embodiment of the present invention;

FIG. 4 includes graphs illustrating transmittance and sheet resistancedata, according to an embodiment of the present invention;

FIG. 5 is an example scanning electron microscope image of a carbonnanotube (CNT)-hybrid film, according to an embodiment of the presentinvention;

FIG. 6 is a flow diagram illustrating techniques for forming a cleanednanotube-graphene hybrid film, according to an embodiment of the presentinvention;

FIG. 7 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 8 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 9 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 10 illustrates a step in forming a field effect transistor (FET),according to an embodiment of the present invention;

FIG. 11 illustrates another embodiment of forming a carbon nanotube FET(CNFET), according to an embodiment of the present invention;

FIG. 12 illustrates an embodiment of a dual-gate CNFET, according to anembodiment of the present invention;

FIG. 13 illustrates a step in another embodiment of forming a CM-ET,according to an embodiment of the present invention;

FIG. 14 illustrates a step in another embodiment of forming a CNFET,according to an embodiment of the present invention;

FIG. 15 illustrates a step in another embodiment of forming a CNFET,according to an embodiment of the present invention; and

FIG. 16 is a flow diagram illustrating techniques for forming ananotube-graphene hybrid film substrate, according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

An aspect of the invention includes a carbon nanotube (CNT)-graphenehybrid as a transparent conductor. As detailed herein, an embodiment ofthe invention uses CNTs to improve the performance of graphenetransparent electrodes. For example, an aspect of the invention includesa combination of thin films of nanotube and graphene, where the thinfilms of nanotubes are deposited over graphene or vice versa.Additionally, this combination can be used in coatings for transparentconductors (TCs).

In an aspect of the invention, the electrical performance of thin filmsof nanotube and graphene is improved by depositing thin films ofnanotubes over graphene or vice versa. As detailed herein, thisarrangement decreases the tube-tube resistance by increasing the contactarea through the use of graphene as a bridge. Also, nanotubes act as abridge across the grain boundary scattering sites of graphene, therebyincreasing the graphene conductivity. Additionally, in contrast toexisting approaches, an aspect of the invention includes increasing the‘out of plane’ conductivity of stacked graphene sheets. As noted,nanotubes also act as bridges between stacked graphene sheets, whichresults in higher electron transmission probability between them.

FIG. 1 is a diagram illustrating an example schematic of the carbonnanotube-graphene composite, according to an embodiment of the presentinvention. By way of illustration, FIG. 1 depicts a substrate 102, agraphene film 104 and a nanotube film 106. The components of thisexample schematic and detailed further herein.

FIG. 2 is a diagram illustrating example two-dimensional (2D) resistorlattices, according to an embodiment of the present invention. By way ofillustration, FIG. 2 depicts a rectangular 2D lattice of resistors 202with a corresponding sheet resistance depiction 204, and a perturbed 2Dlattice 206 with a corresponding sheet resistance depiction 208. Asdetailed herein, carbon nanotubes (depicted as the solid lines dispersedthroughout lattice 206) act as a bridge for electrons to move acrossgraphene grain boundaries, thereby decreasing the scattering at thegrain boundaries. Such a system can be modeled as a resistance network,such as depicted in image 208.

FIG. 3 is a graph 302 illustrating a transmittance and sheet resistancedata comparison, according to an embodiment of the present invention. Byway of illustration, FIG. 3 depicts a comparison of CNT-graphene hybridwith CNT films and graphene with respect to transmittance and sheetresistance data. It is shown through this graph that a CNT-graphenehybrid film has a much higher transparency and a low sheet resistance incomparison to an individual thin CNT film or a graphene film. Inparticular, such an advantage is more enhanced at high lighttransparencies.

FIG. 4 includes graphs illustrating transmittance and sheet resistancedata, according to an embodiment of the present invention. By way ofillustration, graph 402 depicts light transmittance through aCNT-graphene hybrid 406 and a two layer graphene sheet 404. Graph 402illustrates that two stacked layers of graphene are less transparentthan a monolayer of CNTs stacked over a layer of graphene. Graph 408depicts relative decrease in sheet resistance at various CNT densitiesused in making a hybrid. Increasing the CNT density in the hybrid filmdecreases its sheet resistance relative to the sheet resistance of asingle layer of graphene. On the other hand, transparency of the hybriddoes not change much (for example, as shown in graph 302).

FIG. 5 is an example scanning electron microscope image of a CNT-hybridfilm, according to an embodiment of the present invention. By way ofillustration, FIG. 5 depicts a scanning electron microscope imageshowing graphene layer 512 transferred over a film of carbon nanotubes514.

FIG. 6 is a flow diagram illustrating techniques for forming a cleanednanotube-graphene hybrid film, according to an embodiment of the presentinvention. Step 602 includes depositing nanotube film over a substrateto produce a layer of nanotube film. A substrate can be solid such asquartz, silicon, etc., or flexible (plastics). A layer of nanotube filmcan include a layer of carbon nanotube, a network of single wall carbonnanotubes, multi-wall nanotube, film of fullerene C60 molecules, sparsenetwork films made up of any other organic or inorganic conductingmaterials such as metal nanowires (silver, gold, etc.), etc.

Depositing a layer of nanotube film over a target substrate to produce alayer of nanotube film can include performing a vacuum filtrationprocess, a spray deposition process, drop casting a nanotube solution, ananotube coating process, a chemical vapor deposition growth process, awater-surfactant solution based nanotube deposition process, using aself-assembled monolayer of amine terminated groups to increase nanotubeadhesion on the substrate, etc.

As used above (and herein), in a vacuum filtration process, CNT solutionin surfactant and water is passed through a filter paper using vacuumsuction. A uniform CNT film is formed on top of the paper. The paper isthen put in contact with a substrate with the CNT film facing thesubstrate. Pressure is applied to have good contact of CNT film with thesubstrate. Additionally, the filter paper is dissolved in organicsolvent such as acetone, N-methylpyrrolidone (NMP), etc., leaving behindCNT film over the substrate.

Also, drop casting involves putting a drop of CNT solution in water ororganic solvents on the desired substrate and then spinning thesubstrate for uniform coating of solution. A final drying step (eitherin air or in a vacuum oven) will remove the solvent. A chemical vapordeposition process involves deposition of metal catalyst particles (Fe,Mo, Co, Cu, Ni, etc.) on the substrate, placing the substrate in a hightemperature chemical vapor deposition (CVD) oven, flowing carbon carriergases (CO, CH4, C2H2, C2H4, ethanol, etc.) at temperatures ranging from500-1000 degrees Celsius to grow carbon nanotubes from the catalystparticles.

Additionally, in a water-surfactant solution based nanotube depositionprocess, a surface is functionalized with self-assembled monolayers (forexample, 3-Aminopropyl triethoxy silane or aminopropyl triethoxy silane(APTES)) which can attract CNTs. Thereafter, the CNT water-surfactantsolution is deposited over substrate. The substrate is then rinsed withsolvents such as water, alcohol, etc., leaving behind a uniform CNTfilm.

Step 604 includes removing impurities from a surface of the layer ofnanotube film not contacting the substrate to produce a cleaned layer ofnanotube film. Removing impurities from a surface of the layer ofnanotube film can include using an acid, solvent and water cleaningtechnique. Such a technique can include the following steps. For removalof organic impurities, several oxidizing agents can be used such ashydrogen peroxide, piranha solution, etc. Solvents such as acetone,isopropyl alcohol, etc. can be used as well. Mild oxygen annealing attemperatures less than 350 degrees Celsius can also be used to removeamorphous organic impurities. Metal contamination can be removed viaacid treatments such as nitric acid, sulfuric acid, etc. followed by athorough water rinse.

Removing impurities from a surface of the layer of nanotube film canfurther includes annealing the layer of nanotube film under reducedpressure to remove any dried solvent. Additionally, a process referredto herein as vacuum annealing can be performed which includes heating asubstrate with CNT film to a temperature of 80-120 degrees Celsius in alow to moderate vacuum. This helps in removing dried organic solventresidues as well as helps in removing the unintentional doping whichmight occur due to acid cleaning treatments.

Step 606 includes depositing a layer of graphene over the cleaned layerof nanotube film to produce a nanotube-graphene hybrid film. The layerof graphene can include a large area sheet of graphene. Also, depositinga layer of graphene over the cleaned layer of nanotube film to form ananotube-graphene hybrid film includes transferring chemical vapordeposition grown graphene using several known transfer processes, suchas polymer assisted transfer. The graphene films can also be directlyobtained from bulk graphite through a scotch tape transfer process.Graphene can also be deposited through solution in the form of dissolvedgraphene oxide. This can be accomplished through spraying the solutionor spinning graphene oxide flakes suspending in a solvent over thesubstrate containing CNTs, and graphene oxide flakes can later bereduced to graphene through gas or solution phase reducing treatments.In addition to graphene oxided flakes dissolved in polar solvents, anexample embodiment of the invention can also include graphene flakesdissolved in organic solvents such as NMP and dimethylformamide (DMF).Such dissolved flakes can also be sprayed directly over CNT films withno need for a post deposition reduction step.

Step 608 includes removing impurities from a surface of thenanotube-graphene hybrid film to produce a cleaned nanotube-graphenehybrid film, wherein the hybrid film has improved electrical performancevia decreasing nanotube resistance by increasing contact area throughuse of graphene as a bridge. Removing impurities from a surface of thenanotube-graphene hybrid film to produce a cleaned nanotube-graphenehybrid film can include using an acid, solvent and water cleaningtechnique. Also, removing impurities from a surface of thenanotube-graphene hybrid film to produce a cleaned nanotube-graphenehybrid film further includes annealing the nanotube-graphene hybrid filmunder reduced pressure to remove any dried solvent. Cleaning removes theimpurities between physically contacted CNT and graphene films.

The techniques depicted in FIG. 6 also include chemically doping thecleaned nanotube-graphene hybrid film to increase conductivity. A CNTfilm can be a mixture of semiconducting and metallic CNTs. The dopingpermanently increases the charge concentration in semiconducting CNTspresent in the film, thereby decreasing the sheet resistance of thenetwork. The doping step also increases the electrical performance ofthe film. Doping the nanotube-graphene hybrid film can include using asolution doping technique. CNTs can be doped in solution before gettingdeposited over the substrate. Similarly, solution suspended grapheneoxide flakes can be doped before getting deposited over CNTs. Thedopants can be acid solutions such nitric acid, sulfuric acid, etc., orthe dopants can be metal-organic compounds which can formcharge-transfer complexes with the sp² bonded carbon atoms in CNT andgraphene. As detailed herein, the resultant structure can appear asnanotubes scattered over (or under) a single (or multiple), large areagraphene sheet reducing the sheet resistance of graphene.

The techniques depicted in FIG. 6 can additionally include repeating thesteps of claim 1 to form a hybrid film with a desired transparency andat least one desired electrical property. Transparency can be changed byincreasing or decreasing the film thickness. For example, a higherthickness has a lower light transparency. The techniques depicted inFIG. 6 need not be carried out in the sequence illustrated in the flowchart. By way of example, an embodiment of the invention can includedepositing the graphene on a target substrate, cleaning the graphenefilm, and depositing a layer of nanotubes over the layer of graphene toform a nanotube-graphene hybrid film.

Doping is preferably conducted in solution phase, although gas phasedoping is also feasible. For solution processes, organic solvents suchas dichlorobenzene, dichloromethane, ethanol, acetonitrile, chloroform,methanol, butanol, among others, are suitable. Doping can beaccomplished via charge transfer from the dopants to thenano-components, for example, interaction of the lone electron pairs ofdoping molecules with the quantum confined orbitals of semiconductornanowires and nanocrystals which affects the concentration of carriersinvolved in charge transport.

With solution phase doping, for example, nano-components can be dopedbefore and/or after their integration into a circuit on a chip.Nano-components can also be doped locally on the chip using techniquessuch as inkjet printing. The doping level along a nanowire, nanotube ora nanocrystal film can be varied by masking certain portions (forexample, contacts) of the nano-component with resist and doping only theexposed portions. For device applications, nanowires can be protectedfrom damage by implementing the doping at an appropriate stage duringprocess integration.

Nanotubes, for example, carbon nanotubes, can be doped either in bulk bysuspension of the nanotubes in a dopant solution, with or withoutheating; or immersing in the dopant solution a substrate supporting thenanotubes. Although carbon nanotubes are used as examples in thefollowing discussions, doping methods of this invention can also beapplied to other semiconducting nanotubes, which may include, forexample, graphene, pentascene, fuellerence, etc., and combinationsthereof.

Interaction of carbon nanotubes with the dopants, for example, viacharge transfer, results in the formation of charged (radical cation)moeities close to the nanotubes. Bulk doping can be achieved by stirringa suspension of the carbon nanotubes in a dopant solution at a preferredtemperature from about 20 degrees Celsius (C) to about 50 degrees C.,with a dopant concentration preferably from about 1 millimole (mM) toabout 10 moles (M). Depending on the specific dopants and solvents,however, concentration ranging from about 0.0001 M to about 10 M may beused with temperatures from about 0 degrees C. to about 50 degrees C.

In general, the extent of doping depends on the concentration andtemperature of the doping medium, and process parameters are selectedaccording to the specific nano-component, dopant and solventcombination, as well as specific application needs or desired devicecharacteristics.

Device doping, that is, doping the nanotube after it has beenincorporated as part of a device structure of substrate, can be achievedby exposing the device or substrate with the nanotube to a dopantsolution. By appropriately masking the nanotube, selective doping ofportions of the nanotube can be achieved to produce desired dopingprofiles along the nanotube. As noted above, dopant concentration ispreferably in the range of about 0.1 mM to about 10 M, more preferablyfrom about 1 mM to about 1 M, and most preferably, from about 1 mM toabout 10 mM, with the solution temperature preferably from about 10degrees C. to about 50 degrees C., and more preferably, from about 20degrees C. to about 50 degrees C. With device doping, the choice ofprocess conditions also depends on compatibility with other materialspresent on the device or substrate. For example, while lower dopantconcentrations tend to be less effective in general, too high aconcentration of certain dopants may result in potential corrosionissues. In one embodiment, the doping is done under a N₂ atmospherewithout stirring or agitation of the solution. However, agitation of thesolution can be acceptable as long as it does not cause damage to thedevice.

As detailed below, FIGS. 7-10 illustrate steps of a method for forming afield effect transistor (FET). Accordingly, FIG. 7 illustrates a step informing a field effect transistor (FET), according to an embodiment ofthe present invention. A gate dielectric 120 such as silicon dioxide, oroxynitride, or a high K material layer is deposited on gate 100, whichis generally a doped silicon substrate. In an embodiment of theinvention, the silicon substrate is degenerately doped. The gatedielectric can have a thickness, for example, from about 1 to about 100nanometers (nm). A nano-component 140, for example, carbonnanotube-graphene hybrid, is deposited on gate dielectric 120 byspin-coating. A resist pattern is then formed on the carbonnanotube-graphene hybrid 140 by conventional lithographic techniques.For example, a resist layer can be deposited over the carbonnanotube-graphene hybrid 140 and patterned by using e-beam lithographyor photolithography. With a positive resist, regions of the resist layerexposed to the e-beam or lithographic radiation are removed by using adeveloper, resulting in a structure with resist pattern shown in FIG. 7.

The resist pattern formed on the carbon nanotube-graphene hybrid mayhave one or multiple separations from about 10 nm to about 500 nm whene-beam lithography is used, and from about 500 nm to about 10 vm withphotolithography. The multiple separations correspond to the line andspace separations resulting from the respective lithographic techniques,and represent separations between adjacent top gates. The availabilityof multiple top gates provides flexibility of individual control fordifferent logic applications, for example, AND, OR, NOR operations.

As shown in FIG. 8, a metal 160, having a thickness ranging from about15 nm to about 50 nm, is deposited on the resist pattern and overportions of the carbon nanotube-graphene hybrid 140. The metal can bePd, Ti, W, Au, Co, Pt, or alloys thereof, or a metallicnanotube-graphene hybrid. If a metallic nanotube-graphene hybrid isused, the metal 160 may include one or more metallic nanotube-graphenehybrids. Other metals or alloys of Pd, Ti, W, Au, Co, Pt, can bedeposited by e-beam or thermal evaporation under vacuum, while metallicnanotube-graphene hybrids can be deposited with solution phasetechniques such as spin coating.

Following deposition of the metal, the structure can be immersed inacetone or N-methylpyrrolidone (NMP) for resist liftoff, a process thatremoves the lithographically patterned resist and the metal deposited ontop by soaking the sample in solvents such as acetone or NMP. Forexample, such solvents can also be referred to generally as resistliftoff components.

As depicted in FIG. 9, the metal portions 162 and 164 remaining on thecarbon nanotube-graphene hybrid 140 form the FET source and drain. Inthis embodiment, the source and drains are formed over a first and asecond region, respectively, of the carbon nanotube-graphene hybrid 140,or more generally, of the nano-component 140. Following resist liftoff,the structure in FIG. 9 with the carbon nanotube-graphene hybrid 140 isimmersed in an organic solution comprising a suitable dopant asdescribed herein in connection with an embodiment of the invention. FIG.10 illustrates the doping molecules bonding to the carbonnanotube-graphene hybrid 140. The doped portion of the carbonnanotube-graphene hybrid 140 (between the metal source and drain) actsas the channel of the FET.

FIG. 11 illustrates another embodiment of forming a carbonnanotube-graphene hybrid FET, or more generally, a FET with a channelcomprising a nano-component such as other semiconductingnanotube-graphene hybrids, nanowires or nanocrystal films After theformation of gate dielectric 120 on substrate 100, metal portions 162and 164 are formed on gate dielectric 120 using a resist liftoff process(not shown) similar to that described for FIGS. 7-10. Metal portions 162and 164, each having a thickness from about 15 nm to about 300 nm, formthe FET source and drain. Metals such as Pd, Ti, W, Au, Co and Pt, andalloys thereof, or one or more metallic nanotube-graphene hybrids can beused for the metal portions 162, 164. A carbon nanotube-graphene hybrid140, or more generally, a nano-component, is then disposed, for example,by spin-coating, over the gate dielectric 120 and the metal portions 162and 164. Blanket doping of the carbon nanotube-graphene hybrid 140 isachieved by immersing the structure in an organic solution comprising asuitable dopant. The dopant molecules bond to the carbonnanotube-graphene hybrid, for example, via charge transfer interactionwith the nitrogen of a dopant (for example, such as detailed herein)donating a lone pair of electrons to the carbon nanotube-graphenehybrid. In this illustration, the portion of the carbonnanotube-graphene hybrid 140 in contact with the gate dielectric 120forms the channel of the FET.

Alternatively, the carbon nanotube-graphene hybrid 140 can beselectively doped through a patterned resist (not shown) that is formedover the carbon nanotube-graphene hybrid 140. The patterned resist maybe formed, for example, by depositing a suitable resist material overthe carbon nanotube-graphene hybrid 140 and patterning usingconventional lithographic techniques. Hydrogensilsesquioxanes (HSQ), adielectric that can be used as a negative resist, may be used for thispurpose. Also, in an aspect of the invention, conventional resistmaterials can be used such as poly(methyl methacrylate) (PMMA), etc.This is made possible due to the use of water soluble dopants such ascerium ammonium nitrate, cerium ammonium sulfate, and rutheniumbipyridyl complex.

FIG. 12 illustrates an embodiment of a dual-gate carbonnanotube-graphene hybrid FET, or more generally, a FET with a channelcomprising a nano-component such as other semiconductingnanotube-graphene hybrids, nanowires or nanocrystal films. After thegate dielectric 120 is formed over the substrate 100, which acts as afirst gate (also referred to as a bottom or back gate), a carbonnanotube-graphene hybrid, or more generally, a nano-component 140 isdeposited on gate dielectric 120. Metal portions 162, 164 are formedover the carbon nanotube-graphene hybrid 140 using a resist liftofftechnique such as that described in connection with FIGS. 7-10. Aftermetal portions 162, 164 are formed (acting as source and drain of theFET), the structure containing the carbon nanotube-graphene hybrid 140and metal portions 162, 164 is covered with a dielectric layer 180,which can be a low temperature oxide (LTO) or a CVD high dielectricmaterial such as hafnium dioxide.

A second gate 200 (also referred to as top or front gate), which caninclude a metal or highly doped polysilicon, is formed over thedielectric layer 180, for example, by first depositing a gate materialover dielectric layer 180 and then patterning to form top gate 200. Withthe top gate 200 acting as an etch mask, the dielectric layer 180 isetched such that only the portion underneath the top gate 200 remains,as shown in FIG. 12. As an example, a dilute hydrofluoric acid (HF) suchas 100:1 HF can be used as an etchant for LTO.

Additionally, the device is immersed in a dopant solution to achievepartial doping of the carbon nanotube-graphene hybrid 140. In this case,the channel includes both the gated undoped region 500 and the two dopedregions 502 and 504. The doped regions 502 and 504 act like the“extensions” of a complementary metal-oxide-semiconductor (CMOS) FET,resulting in reduced contact barrier and improvements in drive currentand transistor switching. The device can be operated by either the topgate 200 or the bottom gate 100, or both. In logic applications, it isdesirable to operate a FET with the top gate configuration for goodalternating current (AC) performance.

As detailed below, FIGS. 13-15 illustrate steps in another embodiment offorming a carbon nanotube-graphene hybrid FET, or more generally, a FETwith a channel comprising a nano-component such as other semiconductingnanotube-graphene hybrids, nanowires or nanocrystal films. After thecarbon nanotube-graphene hybrid or nano-component 140 is deposited ongate dielectric 120, which has previously been formed over substrate100, a patterned resist is formed on the carbon nanotube-graphene hybrid140 using conventional lithographic techniques such as e-beam orphotolithography.

The structure (shown in FIG. 13) containing the patterned resist andcarbon nanotube-graphene hybrid 140 is immersed in an organic solutionincluding a suitable dopant (as detailed herein). The doping moleculesbond to the exposed portions of the carbon nanotube-graphene hybrid 140.Following doping of the nanotube-graphene hybrid 140, a metal layer 160having a thickness ranging from about 15 nm to about 50 nm is depositedover the patterned resist and the doped carbon nanotube-graphene hybrid140. As previously described, Pd, Ti, W, Au, Co, Pt, or alloys thereof,or one or more metallic nanotube-graphene hybrids can be used for metal160. Metallic nanotube-graphene hybrids can be deposited using solutionphase techniques such as spin coating, while electron beam or vacuumevaporation can be used for deposition of other metals or alloys.

Following deposition of the metal, the structure shown in FIG. 14 isimmersed in acetone or NMP for resist liftoff. As shown in FIG. 15,metal portions 162, 164 remaining after resist liftoff form the sourceand drain of the FET. The process of FIGS. 13-15 generates a significantdoping profile difference along the channel of the carbonnanotube-graphene hybrid transistor. Note that in this case, the undopedportion (portion 500 in FIG. 12, for example) of the carbonnanotube-graphene hybrid 140 forms the channel of the FET.

To complete the formation of the FET devices illustrated in FIGS. 13-15,passivation can be performed by covering the respective devices with aspin-on organic material like poly(methyl methacrylate) (PMMA) orhydrogensilsesquioxanes (HSQ)—a low K dielectric layer, or by depositinga low temperature dielectric film such as silicon dioxide. Furtherprocessing of the device is accomplished via metallization for theback-end of the line.

Also, as detailed herein, an aspect of the invention includes using thenanotube-graphene hybrid film as a coating for a transparent conductor.

Another aspect of the invention includes a process to deposit grapheneover CNTs directly through chemical vapor deposition (CVD). The processto deposit graphene over CNTs directly through CVD includes depositingCNT films over thin copper foils (nickel substrate can be used for thispurpose also). This can be done using variety of techniques such asforming a self-assembled monolayer of aminopropyl triethoxy silane(APTES) or any other amine terminated group over copper (the top surfacecan be copper oxide) and then depositing CNTs from an aqueous solution.This can also be done via depositing CNTs over copper foil from anorganic solvent such as dichloroethane/benzene/N-Methyl-2-pyrrolidone(NMP), etc. The solvent vaporizes, leaving behind CNT filmsAdditionally, depositing CNTs can also be accomplished via spraying CNTsolutions and letting them dry.

The copper foil with as-deposited CNT film is placed in a CVD furnace togrow graphene (accordingly to techniques widely known by one skilled inthe field). Briefly, hydrocarbon gases such as methane, ethylene,acetylene, carbon monoxide, etc. acting as the carbon carriers getdissociated over copper atoms at a high temperature, saturating theatoms with carbon, and eventually causing the formation of graphene.This process is known to be self-limiting; that is, after formation of afirst layer of graphene, the process automatically stops and no furthercarbon layers are formed. The resulting product after CVD is graphenesheets grown around CNTs. At a high temperature, because there are noother residues, this process results in a very good contact between CNTand graphene.

Further, the CNT-graphene hybrid film is transferred over the desiredsubstrate. This can be achieved in a variety of way. Merely by way ofexample, a polymer layer can be spun over copper foil, the polymercontacting the CNT-graphene films. Thereafter, copper is chemicallyetched using FeCl₃ or any other copper solvent. The polymer layer leftfloating in the chemical bath is then cleaned with deionized (DI) water(and also, if needed, with dilute acid solutions to completely removecopper and remaining Fe). The polymer is then transferred onto asubstrate of choice (with graphene-CNT layer facing the substrate). Thepolymer layer is then dissolved in an organic solvent such as acetone,NMP, etc., leaving graphene-CNT film on the substrate.

Additionally, in an embodiment of the invention, multiple sheets ofCNT-graphene hybrid material can be transferred over each other byrepeating the above-detailed steps.

FIG. 16 is a flow diagram illustrating techniques for forming ananotube-graphene hybrid film on a substrate, according to an embodimentof the present invention. Step 1602 includes depositing nanotube filmover a metal foil to produce a layer of nanotube film. As describedherein, depositing a layer of nanotube film over a metal foil to producea layer of nanotube film can include forming a self-assembled monolayerof an amine terminated group over a copper foil and depositing ananotube film from an aqueous solution, depositing a nanotube film overcopper foil from an organic solvent, spraying a nanotube film solutionon the metal foil and allowing the solution to dry, etc.

Step 1604 includes placing the metal foil with as-deposited nanotubefilm in a chemical vapor deposition furnace to grow graphene on thenanotube film to form a nanotube-graphene hybrid film. Placing the metalfoil with as-deposited nanotube film in a chemical vapor depositionfurnace to grow graphene includes saturating copper atoms with a carbongas at a temperature that causes formation of graphene.

Step 1606 includes transferring the nanotube-graphene hybrid film over adesired substrate. The techniques of FIG. 16 can also include repeatingsteps 1602, 1604 and 1606 to form multiple sheets of thenanotube-graphene hybrid film.

Additionally, as detailed herein, an embodiment of the inventionincludes a field effect transistor with a carbon nanotube-graphenehybrid nano-component, which includes a gate, a gate dielectric formedon the gate, a channel comprising a carbon nanotube-graphene hybridnano-component formed on the gate dielectric, a source formed over afirst region of the carbon nanotube-graphene hybrid nano-component, anda drain formed over a second region of the carbon nanotube-graphenehybrid nano-component to form a field effect transistor.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may bemade by one skilled in the art without departing from the scope orspirit of the invention.

What is claimed is:
 1. A nanotube-graphene hybrid film, comprising: asubstrate; nanotube film deposited over the substrate to produce a layerof nanotube film; and graphene deposited over the layer of nanotube filmto produce a nanotube-graphene hybrid film.
 2. The hybrid film of claim1, wherein the nanotube-graphene hybrid film is used as a coating for atransparent conductor.
 3. The hybrid film of claim 1, wherein the layerof nanotube film comprises a layer of carbon nanotube film.
 4. Thehybrid film of claim 1, wherein the layer of nanotube film comprises anetwork of single wall carbon nanotubes.
 5. The hybrid film of claim 1,wherein the nanotube-graphene hybrid film comprises a desiredtransparency.
 6. The hybrid film of claim 1, wherein the layer ofnanotube film comprises a layer of nanotube film removed of impuritiesfrom a surface thereof.
 7. The hybrid film of claim 1, wherein thenanotube-graphene hybrid film comprises a nanotube-graphene hybrid filmremoved of impurities from a surface thereof.